Breaking free: Small phages inhibit murein synthesis to lyse their host.

다국어 초록 (Multilingual Abstract)

Bacteriophages must destroy the bacterial cell wall to lyse their host and release their progeny. There are at least two distinct mechanisms by which phages destroy the cell wall, the choice of which is determined by their genome size. Bacteriophages with large genomes encode a holin-endolysin system. In the prototypic λ model, the holin accumulates in the cell membrane and the endolysin accumulates in the cytoplasm. At a genetically programed time, the holin forms a membrane lesion to release the endolysin into the periplasm where it can degrade the cell wall and cause lysis. In contrast, bacteriophages with small genomes can only afford to encode a single lysis protein. Three unrelated single protein lysis systems are known: the E protein from &phis;X174, and the L and A₂ proteins from MS2 and Qβ, respectively. No cell wall degrading activity has been associated with any of these phages, indicating that their lytic mechanism is distinct from the holin-endolysin system of the larger phages.
The purpose of the work described in this dissertation was to determine the lytic mechanism of &phis;X174. Working from the premise that E targets a host protein to promote lysis, we took a genetic approach to identify this target and isolated dominant mutations in <italic>Escherichia coli mraY</italic> that confer E-resistance. MraY catalyzes the formation of the first lipid intermediate in cell wall synthesis. The isolation of these mutants suggested a model in which E functions similarly to the antibiotic mureidomycin and induces lysis by inhibiting cell wall synthesis at the MraY-catalyzed step. Additional physiological and biochemical experiments demonstrated that cell wall synthesis is indeed inhibited upon <italic>E</italic> expression and that E inhibits MraY <italic>in vivo</italic> and <italic>in vitro</italic>. To determine if other small bacteriophages share the E-induced lysis mechanism, we performed a similar biochemical and genetic analysis of the mechanism of Qβ A₂-induced lysis. This analysis demonstrated that A₂ also functions as a cell wall synthesis inhibitor, but instead of targeting MraY, it targets the MurA reaction, the first and committed step of cell wall synthesis. Cell wall synthesis inhibition therefore appears to be a general lysis strategy employed by small bacteriophages.